Conventionally, in this type of electromagnetic flow meter, an excitation current with a polarity that switches alternatingly is supplied to an excitation coil that is disposed so that the direction wherein the magnetic field thereof is produced is perpendicular to the direction of flow of a fluid flowing within a measuring tube, and a signal EMF that is produced between a pair of electrodes that are disposed within the measuring tube, perpendicular to the magnetic field that is produced by the excitation coil, is detected, and this signal EMF that is produced between the electrodes is differentially amplified to be an AC flow rate signal, where this AC flow rate signal is further amplified, sampled, and subjected to the signal processing to produce a measured flow rate.
This type of electromagnetic flow meter can be broadly categorized into battery-type electromagnetic flow meters, two-wire-type electromagnetic flow meters, and four-wire-type electromagnetic flow meters, based on differences in the driving power supply systems. Additionally, these electromagnetic flow meters have the property of having high measurement accuracy because the signal EMF that is produced between the electrodes is larger in accordance with the flow of the excitation current.
Because the four-wire-type electromagnetic flow meter has electric power supplied to the electromagnetic flow meter through two power supply wires that are separate from the two signal wires, it is possible to increase the excitation current flowing in the excitation coil regardless of the flow rate being measured. In contrast, in the two-wire-type electromagnetic flow meter, electric power that has a self-linking effect is generated from the 4 to 20 mA electric current signal that is sent through the two signal wires, and thus it is not possible to increase the excitation current that can flow in the excitation coil. Moreover, in the battery-type electromagnetic flow meter, the power for driving depends on the built-in battery power supply, and thus the excitation current, of necessity, must be small.
In this way, the excitation current in a two-wire-type electromagnetic flow meter or in a battery-type electromagnetic flow meter is a small electric current when compared to that of the four-wire-type electromagnetic flow meter, and thus the signal EMF that is obtained between the electrodes is smaller. Because of this, the gain in the signal amplifying circuit in the two-wire-type electromagnetic flow meter and the battery-type electromagnetic flow meter is set so as to be large compared to that of the four-wire-type electromagnetic flow meter. Additionally, the signal EMF that is produced between the electrodes has a magnitude that increases with an increase in the flow rate (the speed of flow) of the fluid being measured, and thus switching the gain of the signal amplifying circuit in accordance with the flow rate, or in other words, having the gain the low when the flow rate is high and having the gain the high when the flow rate is low, makes it possible to increase the accuracy depending on the range of the flow rate being measured.
FIG. 10 illustrates schematically an electromagnetic flow meter that is provided with a function for switching automatically the gain of the signal amplifying circuit in accordance with the flow rate (See, for example, Japanese Unexamined Patent Application Publication H6-258111). In this figure: 100 is a detecting device for receiving a magnetic excitation electric current Iex, applying a magnetic field to a fluid that flows within a measuring tube 1C, and detecting the signal EMF that is produced; and 200 is a converting device for not only applying the magnetic excitation electric current Iex to the detecting device 100, but also processing the signal EMF from the detecting device 100 to measure the flow rate of the fluid flowing within the measuring tube 1C.
In this electromagnetic flow meter, the converting device 200 has a differential amplifying circuit 2; an AC amplifying circuit 3; a sample hold circuit 4; a DC amplifying circuit 5; an A/D converting circuit 6; a processing portion 7; and a magnetic excitation circuit 8. Additionally, the detecting device 100 is provided with: a magnetic excitation coil 1D arranged so that the direction in which the magnetic field thereof is produced is perpendicular to the direction of flow of the fluid flowing within the measuring tube 1C; and a pair of electrodes 1A and 1B disposed perpendicular to the direction of flow of the fluid flowing within the measuring tube 1C and to the direction of the magnetic field produced by the magnetic excitation coil 1D.
In this electromagnetic flow meter, the magnetic excitation circuit 8 outputs a square wave AC magnetic excitation electric current Iex of a specific frequency based on an instruction from the processing portion 7. The magnetic excitation coil 1D is excited magnetically by the magnetic excitation electric current Iex from the magnetic excitation circuit 8, to produce a magnetic field, where the magnetic field that is produced is applied to the fluid flowing within the measuring tube 1C. This produces a signal EMF between the electrodes 1A and 1B with an amplitude that is in accordance with the speed of flow of the fluid. The signal EMF that is produced between the electrodes 1A and 1B is inputted into the differential amplifying circuit 2.
The differential amplifying circuit 2 performs differential amplification on the signal EMF produced between the electrodes 1A and 1B, to produce an AC flow rate signal. This AC flow rate signal is amplified by the AC amplifying circuit 3, and applied to the sample hold circuit 4. The sample hold circuit 4 samples the AC flow rate signal that is amplified by the AC amplifying circuit 3, to produce a DC flow rate signal. This DC flow rate signal is amplified by the DC amplifying circuit 5, and applied to A/D converting circuit 6. The A/D converting circuit 6 converts into a digital signal the DC flow rate signal that has been amplified by the DC amplifying circuit 5, and sends it to the processing portion 7. The processing portion 7 calculates the flow rate of the fluid flowing within the measuring tube 1C from the digital signal from the A/D converting circuit 6, and outputs the calculated flow rate as the measured flow rate. Additionally, the processing portion 7 switches the gain in the AC amplifying circuit 3 in accordance with the calculated measured flow rate, that is, switches the gain that is applied to the AC flow rate signal from the differential amplifying circuit 2. In this case, there is a low gain when the measured flow rate is high, and a high gain when the measured flow rate is low.
However, in the conventional electromagnetic flow meter illustrated in FIG. 10, the gain of the AC amplifying circuit 3, which is a stage prior to the sample hold circuit 4, is switched, and thus when low frequency noise, or the like, is produced when there is a high gain when the flow rate is low (that is, when low frequency noise is produced in, for example, a case wherein a solid object within the flow strikes the measuring electrode), there is an increased likelihood that the operational amplifier in the AC amplifying circuit 3 will become saturated, and thus there is the risk that there will be error in the measured flow rate due to the occurrence of saturation.
The present invention was created in order to solve the problem as set forth above, and the object thereof is to provide an electromagnetic flow meter able to increase the accuracy of the measured flow rate, without producing saturation.